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I moved into my house about five years ago and was told that it had a waterproofed basement. The inspection didn’t note any issues with the basement, including the waterproofing system.

After moving in, we noticed that some areas of the basement had what looked to us at the time like a waterline, where it was black about 6 inches to one foot up the wall. We thought that water must have gotten in at some point (weird) and tried to scrub away as much as we could with a bleach based cleaner.

Fast forward five years later to today, when we noticed that it had started to return in some areas, primarily where air is more obstructed (behind a shelf, behind the washer, etc). This time, it more clearly seemed like it was coming up from the ground somehow.

Here’s a pic of what we’re dealing with:

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I’m at a loss as to what to do. Is this harmful? Should I attempt to clean it myself? Should the waterproofing system be preventing it? Where could it even be coming from?

FWIW, we have a dehumidifier in the affected space, and a vent fan. So it does get humid down there, but I thought we were kind of already doing the things we could do…

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It definitely looks like mold to me.

It's hard to say where the water is coming from that is causing that. Water could be running down and then wicking up, it could be making it's way in at the bottom of the slab/footer, or it might even be a high water table.

Waterproof systems are basically marketing. They might stop it for a time or slow it, but the only way to reliably keep a basement dry is to have a perimeter drain and parged exterior; this allows water to be collected and sent elsewhere and prevent water from pushing throw the wall. This is costly if not done when the house is constructed.

I can't quite tell what the wall is made of.

  • If it is drywall/sheetrock with a textured paint, I would remove about a foot or two from the bottom and see if I could identify where the water is coming in. This will prevent the sheeting from wicking water up. Wear a mask when doing this work to mitigate the mold and dust.
  • If it is concrete/cement with a water proof masonry paint, it is obviously not working. I would spray it with a diluted bleach and scrub it with a brush. This will clear the surface mold.

Resolving the issue means determining the cause first. You can't waterproof effectively from the interior, so fixing it might mean spending the time and/or money to trench, parge, and put in a drain.

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    That looks like some kind of FRP wall board panel to me. (Not that it really matters w.r.t. the advice given.)
    – Huesmann
    Sep 11, 2023 at 12:02
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Is this harmful?

No. Fungi are everywhere. You are breathing spores right now.

Should I attempt to clean it myself?

Spray bleach and rinse.

Should the waterproofing system be preventing it?

Maybe. Waterproofing a basement is really hard. But it's easy to spray bleach.

Where could it even be coming from?

Fungi will grow anywhere there's humidity. You could install a dehumidifier, use water-blocking paint, or just live with the fact that you'll spray the basement with bleach once a year.

In my opinion, bleach is the best, fastest, cheapest. Triple win in my book.

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